This invention relates to a semiconductor device having electric elements such as semiconductor elements mounted thereon and a method for manufacturing the same.
There are many kinds of semiconductor devices which have functional elements such as semiconductor elements, signal and/or power wirings, and passive elements such as resistors and capacitors. Such devices include hybrid ICs and various IC packages. One of such devices comprises what is called a plastic package which can be made by automation by use of lead frame and resin sealing and a board in the form of an alumina substrate having Ag--Pd wirings formed thereon by screen printing, a laminated ceramics board having multi-layered wirings or a printed wiring board being mounted in the plastic package. Such a device can accommodate a plurality of elements therein. Japanese Unexamined Patent Publication 62-185349 discloses a plastic package which has resistors and film wiring layers formed on an insulating film formed on die pads of a lead frame made of FeNi alloy to increase density.
Recent electronic parts such as semiconductor packages are becoming more densely integrated, lighter, thinner, shorter and smaller and less expensive. But with this tendency a semiconductor element tends to heat up easily. Thus, a semiconductor package having a high heat is dissipation required. Demands for plastic packages are especially high because they are mass-productive and less costly.
But the aforementioned plastic package having an alumina wiring board built therein has a problem that it is difficult to produce thin and fine wirings. With plastic packages containing a printed wiring board, besides the difficulty in fining the wirings and the softening of adhesives when mounting the element, especially during wire bonding, its low long-term reliability was the problem. Further, the technique disclosed in Japanese Unexamined Patent Publication 62-185549 for achieving high density is not adapted for mass-production because it is difficult to form insulating films having high insulating properties on metals with high yield.
The prior art packages had the problem of poor heat dissipation and thus could not meet the requirement for higher speed and higher density.
In view of these problems, we conceived of forming an insulating film on the surface of a rolled substrate made of copper or copper alloy (containing 50 wt % or less of non-copper elements), which is gaining popularity as a lead frame material these days, mounting functional elements on the insulating film or the exposed surface of the substrate, and further providing film wirings as relays to external leads and noise-removing film resistors and film capacitors on the insulating film. In this case, the insulating film is made of an inorganic material and is provided without using any adhesive. Copper or copper alloy has a high heat dissipating capacity and has a thermal expansion coefficient similar to that of the molded resin. Thus, resin can be molded stably. But if the insulating film is a laminated film which is laminated using an adhesive, not only will the heat dissipating capacity drop, there will also arise problems such as decreased heat resistance and difficult thining of the package. Use of a resin film and/or anorganic adhesive can aggravate the long-term reliability of the package as well.
On the other hand, if an inorganic insulating film is provided directly on the substrate of copper or copper alloy, the following problems will arise:
1) While the thermal expansion coefficient of such a substrate is 17.times.10.sup.-6, the thermal expansion coefficient of an inorganic insulating film is typically 10.times.10.sup.-6. The difference is rather great. On the other hand, the film forming temperature during the film forming step has to be kept high for better adhesion and reactivity. When the film formed is cooled thereafter, it is subjected to an excessive stress due to the difference in the thermal expansion coefficient. Due to this stress, the film may sustain cracks or its moisture absorption speed may be accelerated, worsening the resistance to moisture. PA1 2) Copper and copper alloy surfaces oxidize easily and a very brittle oxide film forms. Thus, water taken by moisture absorption into the interface between the insulating film and the board will oxidize the surface of the board and the resulting oxide layer will be broken and peel off the insulating layer under the stress which acts on the insulating film. PA1 3) The surface of the rolled metal sheet is rather rough, i.e. Rmax=0.2 micron or more. It is difficult to cover the surface with a thin film and thus provide sufficient insulation. PA1 IMI=max (IO1, IN1), greater one of IO1 and INI: PA1 I11: maximum absorption height by Si--H bond in a wave number range of 2150-2350 cm.sup.-1 PA1 I21: maximum absorption height by Si--OH bond in a wave number range of 860-980 cm.sup.-1 PA1 1IO1: maximum absorption height by Si--O bond in a wave number range of 1020-1200 cm.sup.-1 PA1 INI: maximum absorption height by Si--N bond in a wave number range of 840-920 cm.sup.-1 PA1 IM2=max (IO2, IN2), greater one of IO2 and IN2: PA1 112: maximum absorption height by Si--H bond in a wave number range of 2150-2350 cm.sup.-1 PA1 I22: maximum absorption height by Si--OH bond in a wave number range of 860-980 cm.sup.-1 PA1 I02: maximum absorption height by Si--O bond in a wave number range of 1020-1200 cm.sup.-1 PA1 IN2: maximum absorption height by Si--N bond in a wave number range of 840-920 cm.sup.-1 PA1 IM3=max (I03, IN3), greater one of I03 and IN3: PA1 I13: maximum absorption height by Si--H bond in a wave number range of 2150-2350 cm.sup.-1 PA1 I23: maximum absorption height by Si--OH bond in a wave number range of 860-980 cm.sup.-1 PA1 IO3: maximum absorption height by Si--O bond in a wave number range of 1020-1200 cm.sup.-1 PA1 IN3: maximum absorption height by Si--N bond in a wave number range of 840-920 cm.sup.-1
An object of the present invention is to provide a cost-effective plastic package type semiconductor device which has a good heat dissipating capacity and which permits high integration by eliminating the above problems 1)-3), which impair insulation of the insulating film on the board and long-term reliability.
In accordance with the present invention, there is provided a semiconductor device comprising a rolled metal substrate made of copper or a copper alloy composed of copper as a matrix and at least one other element in an amount of 50 wt % or less, an insulating film provided on a surface of the metal substrate and having a thickness of from 1 microns to 20 microns inclusive, a semiconductor element mounted on the insulating film or on the exposed surface of the metal substrate, and signal and/or power film wirings, and passive elements such as a film resistor or a film capacitor, mounted on the insulating film, the insulating film being a single-layer film made of a mixture of silicon oxide and silicon nitride or a multi-layered film having two or more layers laminated one on another and comprising a silicon oxide layer and a layer made of a mixture of silicon oxide and silicon nitride or a silicon nitride layer.
The metal board used in the present invention should preferably have a surface roughness Rmax of not less than 0.1 micron. This surface roughness may be obtained while rolling the material or by grinding the board surface mechanically or electrically after rolling. It is also preferable to form, on the surface of the metal board, a single-layered metal film made of one selected from Ni, Cr, al, Ti, Au, Pt and Ag or a laminated film or an alloy film made of two or more of the above elements. The film thicknesses of these metal films should be preferably 0.1 to 10 microns.
The signal or power film wirings formed on the insulating film (or on the substrate) by sputtering, vacuum deposition or ion plating have a three-layer structure comprising e.g. an aluminum film as a conductive layer, a nickel layer formed thereon as a layer for preventing diffusion, and an uppermost gold layer on which are performed wire bondings. To form a microscopic wiring pattern, the metal mask patterning or photolithographic technique may be used. By forming such filmy wirings, high integration with fine patterns becomes possible. Further, since the metal substrate and the film wirings are separated from each other by the insulating film, the latter acts as a bypass capacitor which serves to keep the noise low.
Film resistors having a desired resistivity may be formed from a film resistor material such as NiCr, TaN, TiN or CrSiO by sputtering, vacuum deposition or ion plating. For fine adjustment of the resistivity, laser trimming may be used. The film capacitors may be formed from Ta.sub.2 O.sub.5 by sputtering, vacuum deposition or ion plating. Also, as described above, the insulating film between the board and the film wirings may be used as a capacitor. These passive elements such as resistors and capacitors serve to dampen signal ringings which can pose a problem when transmitting high-speed signals. This permits speedups of operation of the semiconductor package.
The insulating film of silicon compound may be any one selected from (1) a single-layered silicon oxynitride film containing nitrogen preferably in the atomic ratio to Si of 0.3 to 0.8, (2) a composite film made up of two or more layers including silicon oxide layer and silicon oxynitride or silicon nitride layer containing nitrogen preferably in the atomic ratio to Si of 0.3 or more, and (3) a composite film formed of two or more layers including silicon oxide layer and silicon oxynitride layer, its nitrogen content increasing or decreasing continuously in the direction of thickness of the film so that the maximum nitrogen content is 0.3 or more in the atomic ratio to Si.
Throughout the specification and the claims, the term "silicon oxynitride" is used to mean "a mixture of a silicon oxide compound and a silicon nitride compound".
Preferably, the silicon oxynitride layers, silicon oxide layers and silicon nitride layers should have as a intrinsic stress of the film a compressive stress or a tensile stress of 8.times.10.sup.9 (dyne/cm.sup.2) and more preferably have a compressive stress of 8.times.10.sup.9 (dyne/cm.sup.2) or less or a tensile stress of 3.times.10.sup.9 (dyne/cm.sup.2) or less. The intrinsic stress of the film herein referred to represents a value obtained by subtracting the thermal stress resulting from the difference in thermal expansion coefficient between the board and the film from the total residual stress of the film formed on the board. The total residual stress can be measured in several ways. For example, as a destructive test, the stress can be calculated from the warpage of the film peeled off the substrate. As a non-destructive test, the stress can be calculated from the warpage of the board carrying the film.
Any of the above said insulating films (1)-(3) contains hydrogen. The single-layered film (1) contains combined hydrogen preferably in an amount which satisfies at least one of the following formula (1) and (2) at the absorption peak of infrared absorption spectrum: EQU 0.001.ltoreq.I11/IM1.ltoreq.0.3 (1) EQU 0.01.ltoreq.I21/IM1.ltoreq.1.5 (2)
wherein,
The composite insulating film (2) should contain, in the silicon compound in each layer, combined hydrogen preferably in an amount which satisfies at least one of the following formula (3) and (4) at the absorption peak of infrared absorption spectrum: EQU 0.001.ltoreq.I12/IM2.ltoreq.0.3 (3) EQU 0.01.ltoreq.I22/IM2.ltoreq.1.5 (4)
wherein,
The composite film (3) should contain, in the silicon compound in each layer, combined hydrogen preferably in an amount which satisfies at least one of the following (5) and (6) at the absorption peak of infrared absorption spectrum: EQU 0.001.ltoreq.I13/IM3.ltoreq.0.3 (5) EQU 0.01.ltoreq.I23/IM3.ltoreq.1.5 (6)
wherein,
Further, the insulating film may contain one element or two or more elements selected from a group consisting of F, C, B, S, P, A1, Se, As, Ge, Ga, Zn, Te, Sb, Sn and In in the amount of 10% of less in terms of the atomic ratio to Si. Also, the insulating film may have at a superficial layer and/or a lowermost layer moisture-resistant protective films made of non-conductive carbon, SiC, Al.sub.2 O.sub.3, Zr.sub.2 O, A N, Bn or a combination thereof.
In order to form such an insulating film on the board evenly with good adhesion, according to the present invention, oxides on the surface of the metal board is firstly removed by plasma cleaning in the atmosphere of Ar gas, H.sub.2 gas or a mixture thereof, under the following conditions: board temperature: 350 or less, atmospheric gas pressure: 0.1-50 Tort, discharge output: 0.1-10 W/cm.sup.2, discharge time: one minute or more. An insulating film is then formed on the board by vapor phase development with the plasma CVD (P-CVD) or the CVD technique, using an Si source gas, a nitrogen source gas and an oxygen source gas as materials, under the following conditions: board temperature: 350.degree. C. or less, gas pressure when forming film: 0.1 Torr or more and not more than the atmospheric pressure, flow ratio of nitrogen and oxygen source gases to Si source gas: 0.5-30, preferably 0.5-10. The Si source gas may be Sin H.sub.2n+2 (n=1, 2, 3, 4), Si (C.sub.2 H.sub.5 O).sub.4, or other organic Si gases. The N source gas may be N.sub.2 or NH.sub.3. The O source gas may be O.sub.2, O.sub.3, N.sub.2 O, NO, NO.sub.2 or a mixture thereof.
As another method, after removing oxide layers on the surface of the metal board by electrochemical treatment, the insulating film may be formed on the metal board with the plasma CVD technique, using SinH.sub.2n+2 (n=1, 2, 3, 4) or Si (C.sub.2 H.sub.5 O) as an Si source gas, NH.sub.3 or N.sub.2 as an N source gas, and one or more gases selected from N.sub.2 O, NO, NO.sub.2, O.sub.2 and O.sub.3 as oxygen source gas, under the following conditions: metal board temperature: 350.degree. C. or less, preferably 200.degree.-350.degree. C., gas pressure at plasma discharge for pretreatment: 0.1-30 Tort, preferably 0.5-50 Torr, applied high-frequency output: 0.1-10 w/cm.sup.2, preferably 0.5-10 w/cm.sup.2, discharge time: one minute or longer, preferably five minute or longer, gas pressure when forming film: 0.1-5 Tor4, gas flow ratio: Fs/(Fn+Fo).ltoreq.2, preferably 0.1.ltoreq.Fs/(Fn+Fo).ltoreq.2, (wherein Fs is a gas flow rate of Si source gas, Fn is a gas flow rate of N source gas and Fo is a gas flow rate of O source gas), high-frequency output density; 0.1-2.0 w/cm.sup.2, distance between the board and electrodes: 10-40 mm, preferably 20-40 mm, and board temperature: 350.degree. C. or lower, preferably 200.degree.-e50.degree. C.
Insulating films according to the present invention can be formed with other methods such as high-frequency magnetron sputtering, laser deposition, ion plating, vacuum deposition, ECR plasma CVD, laser CVD or optical CVD.
A silicon oxide film has a good strength as a material and has small Young's modulus and Poisson's ratio, which can increase stresses. Thus, cracks hardly develop but it readily absorbs moisture. On the other hand, unlike silicon oxide, silicon nitride is known to scarcely absorb moisture though cracks develop easily.
The silicon oxynitride film used in the present invention has both the advantages of the above two materials. That is, cracks hardly develop and moisture absorption, which is the cause of deterioration and peeling of the film, hardly occur. This is also true of the film formed by laminating a silicon oxide layer and a silicon oxynitride layer or a silicon nitride layer.
The present inventors studied the mechanism on how cracks develop and have discovered that in most cases, cracks develop on the silicon compound insulating film provided on the copper or copper alloy board if the film is subjected to tensile stress and more particularly such cracks develop only if the intrinsic stress during the film forming step is an excessive tensile stress. We studied the relation between the intrinsic stress and crack formation for silicon oxide, silicon oxynitride and silicon nitride films having a thickness of 20 microns or less. No cracks were observed if the tensile stress as the intrinsic stress was 8.times.10.sup.9 dyne/cm.sup.2 or less or the intrinsic stress was a compressive stress. From this fact, we have found that crack formation can be avoided if the intrinsic stress during the film forming step is a small tensile stress or a compressive stress.
In a temperature cycle test, one of the reliability tests of a semiconductor device, stress change on the film is applied repeatedly. In such a condition, cracks develop more readily. We discovered that in order for the film to withstand such a repeated fatigue test, it is necessary to control the compressive stress to 8.times.10.sup.9 dyne/cm.sup.2 or less or the tensile stress to 3.times.10.sup.9 dyne/cm.sup.2 or less.
As a result of further studies and experiments, we also found that by controlling the amount of hydrogen contained in the film, the intrinsic stress during the film forming step can be a small tensile or compressive force. By measuring and controlling the the amount of combined hydrogen by infrared absorption spectrum, the intrinsic stress can be controlled in such a manner that cracks will not develop on the film. Based on these findings, we succeeded in establishing the method of determining whether the film may develop cracks or not.
Now we shall describe what influences the hydrogen contained in the film have on the stress of the film.
As is well-known, a silicon oxide film or a silicon oxynitride film formed with a vapor phase technique such as the CVD or P-CVD technique has an amorphous structure having no long-distance order as a crystal lattice. But it is crystalline if one see a short-distance order, i.e. one unit cell at minimum. For example, each cell of a silicon oxide film comprises Si and 0 and maintains the shape of a substantially right tetrahedron. But the adjacent cells are bonded together in a disturbed manner due to dangling bonds or the bonds between redundant atoms. If there exist bonding hands other than the Si--O or Si--N bonds or dangling bonds, such portions will be locally subjected to strain. As a result, the entire film is subjected to stress. Taking these phenomena into due consideration, the present inventors studied in detail the relation between the content of the hydrogen atoms as redundant atoms in the film and the internal stress of the film. As a result, we found the optimum range of the ratio of the absorption peak by the Si--H bond or Si--OH bond to the absorption peak by the Si--O or Si--N bond to keep the intrinsic stress to a difficult-to-crack value, which is represented by the above-listed relational expressions 1-6.
As described above, since the content of hydrogen in the film can be controlled by measuring the amount of bound hydrogen by the infrared absorption spectrum, the film can be protected reliably against excessive stress during the film forming step. The insulating film thus obtained is free of cracks and of high quality.
For similar reasons, one or more elements selected from F, C, B, S, P, Al, Se, As, Ge, Ga, Zn, Te, Sb, Sn and In may be added in addition to hydrogen, in an amount of 10% or less in terms of the ratio to Si atoms.
A single-layered film made of one of Ni, Cr, Al, Ti, Au, Pt and Ag or a laminated film or alloy film made of two or more of the above said metals may be formed beforehand on the metal substrate as a protective layer, which serves to prevent the formation of a brittle oxide layer on the substrate of copper or copper alloy. Even if there are parts on the metal substrate where the insulating film is not formed, the metal film serves to prevent deterioration by moisture absorption at the exposed part between the metal substrate and the edge of the insulating film. If its film thickness is 0.1 micron or less, the film cannot cover the substrate sufficiently and thus is not effective. On the other hand, if the film thickness is 10 microns or more, the film forming cost will be too high to be practical. Also, the adhesion of such a film to the substrate is so weak that it can peel off easily.
Now, we shall discuss in detail the conditions and effects in the method for forming a high-performance insulating film made of a silicon compound on a substrate made of a rolled metal and having a rough surface with the plasma CVD or CVD technique.
By heating the substrate and removing any oxides on the surface layer by plasma dischage in the atmosphere of Ar gas, H.sub.2 gas or their mixture, the surface of the substrate is cleaned and the adhesion of the film improves. The substrate temperature in this step should be 350.degree. C. or less. Otherwise, the heating temperature will exceed the softening temperature of the copper and copper alloy contained in the substrate. In order to clean the substrate surface sufficiently, the plasma discharge time should be one minute or longer, though this is also dependent upon the gas pressure and applied output.
As material gases for forming an insulating film of silicon compound containing oxygen and nitrogen, Si source gases, N source gases and O source gases as described above are used. The content of hydrogen in the film can be controlled by adjusting the flow ratio of these gases or by adding hydrogen gas as a reaction gas.
The gas pressure when forming the film should be 0.1 Torr or more and not more than the atmospheric pressure. If less than 0.1 Torr, the film forming rate is too small to be practical. Further, since the film cannot be formed with good covering rate on a rolled metal substrate having a rough (Rmax.gtoreq.0.1 micron) surface, the insulating property of the films is no good. This is because the gas average free path depends on the degree of ambient vacuum and the lower the gas pressure, the longer the average free path and thus the lower the throwing power of the film to the substrate.
The contents of oxygen and nitrogen in the film may be adjusted by adjusting the flow ratio of source gases. It was necessary to set the upper limit for the gas flow ratio Fs/(Fn+Fo) to avoid insufficient specific resistance of the film resulting from insufficient content of oxygen and nitrogen in the film and to avoid the content of hydrogen in an excessive amount. On the other hand, the lower limit was provided to prevent the development of cracks while forming the film due to shortage of hydrogen in the film.
The film thickness of the insulating film is restricted to 1-20 microns because if less than 1 micron, even if the film covers the substrate surface with high throwing power, the insulation yield would be bad on a rolled metal plate having a rough surface. If more than 20 microns, not only will the cost too high, but such a film can develop cracks easily. The film thickness should preferably be 3 to 10 microns inclusive.
According to the present invention, an insulating film having good insulating properties as well as high resistance to cracks, moisture and peeling is formed on a rolled metal substrate made of copper or copper alloy. On this film, film wirings and passive elements for removing noise are provided. Further, active elements are mounted on the insulating film or on the exposed surface of the substrate. Thus, this construction in a plastic package form contributes to improved heat dissipation, finer wirings and further thinning of the device. Such a device satisfies the requirements for high-scale integration, higher performance, more compact size and improved productivity, and lower cost.